In recent years, water-borne coating compositions and water-borne ink compositions that contains only extremely low amount of organic solvents are widely used in the fields of coating compositions and ink compositions as a measure for preventing pollution. In Europe, effluent control of VOC has been under enforcement for a long time. Therefore, shift to water-borne coating compositions or water-borne ink compositions from solvent-borne coating compositions or solvent-borne ink compositions will be accelerated from now on. In order to attend to such needs for water-borne systems, it has been desired to incorporate various compounds, which have been used in a solvent-borne coating composition and a solvent-borne ink composition so far, in a water-borne coating composition or a water-borne ink composition without any property deterioration.
As one example of such compounds that has been demanded to use in water-borne coating compositions or water-borne ink compositions, ultrafine barium sulfate particles can be mentioned. Ultrafine barium sulfate particles mixed in solvent-borne coating compositions or solvent-borne ink compositions improve color saturation and gloss of color pigments, such as carbon black, or improve orientation of aluminum flakes or pearl pigments, which have been used in metallic coating compositions (See the below Patent Documents 1 to 3).
However, barium sulfate originally has low affinity for resins. Thus, for incorporating in solvent-borne coating compositions or ink compositions, the surface of barium sulfate has conventionally been treated with an alkali, so that pigment pH value of the barium sulfate should be 9 or higher (See the below Patent Document 4). Barium sulfate is not sufficiently dispersed in water-borne coating compositions or water-borne ink compositions, and has low affinity for resins for water-borne coating compositions or water-borne ink compositions. Thus, if ultrafine barium sulfate particles alone are mixed with a resin, the resultant coated film shows insufficient transparency and insufficient gloss. Water-borne coating compositions or water-borne ink compositions that contain such ultrafine barium sulfate particles show insufficient improvement of color saturation and gloss of colored pigments, such as carbon black, or insufficient improvement of orientation of aluminum flakes or pearl pigments, which have been used in metallic coating compositions.
A treatment by an organic phosphorous compound is disclosed in the below Patent Documents 5, as an example of ways to treat the surface of an ultrafine barium sulfate particle with an average primary particle diameter of 0.10 μm or less. However, the surface treatment is not for water-borne coating compositions, water-borne ink compositions or the like, but for a treatment to improve the affinity of the ultrafine barium sulfate particle with solvent-borne compositions.    [Patent documents 1] JP 60-79074, A    [Patent documents 2] JP 59-90671, A    [Patent documents 3] JP 5-163447, A    [Patent documents 4] JP 58-120520, A    [Patent documents 5] JP 9-156924, A